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To better understand the contributions of effort on cortical activation associated with motor tasks, healthy participants with varying capacities for isolating the control of individual finger movements performed tasks consisting of a single concurrent abduction of all digits (Easy) and paired finger abduction with digits 2 and 3 abducted together concurrently with digits 4 and 5 (Hard). Brain activity was inferred from measurement using functional magnetic resonance imaging. Effort was measured physiologically using electrodermal responses (EDR) and subjectively using the Borg scale. On average, the Borg score for the Hard task was significantly higher (p=0.007) than for the Easy task (2.9+/-1.1 vs. 1.4+/-0.7, respectively). Similarly, the average normalized peak-to-peak amplitude of the EDR was significantly higher (p=0.002) for the Hard task than for the Easy task (20.4+/-6.5% vs. 12.1+/-4.9%, respectively). The Hard task produced increases in sensorimotor network activation, including supplementary motor area, premotor, sensorimotor and parietal cortices, cerebellum and thalamus. When the imaging data were subdivided based on Borg score, there was an increase in activation and involvement of additional areas, including extrastriate and prefrontal cortices. Subdividing the data based on EDR amplitude produced greater effects including activation of the premotor and parietal cortices. These results show that the effort required for task performance influences the interpretation of fMRI data. This work establishes understanding and methodology for advancing future studies of the link between effort and motor control, and may be clinically relevant to sensorimotor recovery from neurologic injury.

We have recently observed medio-lateral balance adjustments (BA) preceding reflexive stepping elicited by noxious stimulation. While task specific modulation is evident for BA prior to voluntary leg movement, it is unclear whether rapid BA reactions (prior to 'reflexive' stepping) represent a generic response to evoked limb withdrawal or can be modified to suit task-conditions. This study was designed to establish whether the CNS is able to modify rapid onset latency BAs to match task conditions. Reflexive stepping was evoked by applying a noxious stimulus (50 ms stimulus train, 1 ms pulses, 300 Hz, 4 x perceptual threshold) to the plantar surface of the either the left or right foot. Task conditions were varied prior to stimulation by having subjects maintain one of three different static positions: (1) lean left (70% body weight (BW) on left), (2) neutral (50% BW both sides), (3) lean right (70% BW on right). BAs were denoted by centre-of-pressure (CoP) excursions towards the swing foot after the onset of noxious stimulation (average onset latency of 128 ms). There was a significant increase in frequency of occurrence and a significant increase in magnitude of CoP shift when the stimulation was applied to a loaded limb (leaning with 70% BW on the stimulated foot) as compared to an unloaded limb (30% BW). In addition, 78% of loaded trials featured steps taken with the unstimulated foot, which delayed removal of the stimulated foot. Collectively, the results indicate modifiability of the very rapid onset balance adjustments that precede the onset of limb withdrawal revealing complex control of balance exists even over very brief latencies.

The gain of somatosensory afferent paths from the lower limb to the cerebral cortex was investigated during the acquisition of one target location during plantar flexion. Sensory gain was measured as the magnitude of somatosensory evoked potentials (SEPs) following electrical stimulation of a peripheral nerve in the lower limb, and was recorded from the scalp. We hypothesized gain attenuation of SEPs from sensory paths serving the limb segment responsible for target acquisition. SEP gain was studied as subjects plantar flexed about the anide to a target that was 15 degrees beyond the occurrence of a cutaneous stimulus (cue) to the lateral border of the foot. The "cue" was either fixed in one location or could appear at one of three positions in space. SEP gain was tested during practice and with task acquisition. Electroencephalographic (EEG) recordings were made of primary and secondary complexes of cortical SEPs from sural and tibial nerve stimulation, with 30-40 samples averaged per subject-condition. Electromyographic (EMG) records were made of soleus muscle H-reflexes and M-waves. Target acquisition was recorded as percent correct hits. The results showed significant attenuation in sural and tibial nerve primary SEPs with task acquisition when the cue was fixed or varied in movement space (P<0.05). Secondary SEPs from tibial nerve followed this pattern. Spinal H-reflexes only attenuated with movement per se. We conclude that the CNS preferentially reduces the cerebral inflow of sensory information once such a motor task has been successfully acquired.

The afferent origin of the secondary somatosensory evoked potential elicited from stimulation of the sural and tibial nerves was investigated as the limb was cooled. It was hypothesized that the peak of this potential is initiated from primary afferents in the A alpha group. We conclude that the peak of the secondary SEP arises from an afferent source whose diameter is of similar size to that of large diameter A alpha afferents.

The role of plantar pressure sensation in controlling compensatory stepping was explored via hypothermic anesthesia of the foot soles, in 10 healthy young adults. Stepping reactions were evoked by unpredictable platform translation in forward, backward and lateral directions. The findings suggest three specific direction- and phase-dependent roles for the plantar cutaneous afferents: (1) sensing posterior stability limits during initiation of backward steps, (2) sensing and controlling heel-contact and subsequent weight transfer during termination of forward steps, and (3) maintaining stability during the prolonged swing phase of lateral crossover steps.

Movement-related gating of somatosensory evoked potentials in the upper limb is restricted mainly to nerve stimulation supplying the moved limb segment. In the lower limb, this principle may not be followed. Tibial nerve (stimulation at the knee) somatosensory evoked potentials (SEPs) and soleus H reflexes exhibit quite similar patterns of modulation during movement. We hypothesised that movement-related gating of initial SEPs in the leg would be generalised from ipsilateral to contralateral leg movement and that such sensory gating would not be generalised to modalities with no functional relevance to the movement. Somatosensory, visual, and auditory evoked potentials (SEPs, VEPs, and AEPs) were recorded from scalp electrodes during unilateral passive movement. Short-latency tibial nerve SEPs, representing the first cortical components, and soleus H reflexes in both the moved leg and the stationary leg were attenuated compared to non-movement controls (p<0.05). Neither VEPs nor middle latency AEPs were modulated (p>0.05). We conclude that sensory gating occurs during contralateral movement. This gating is absent in other sensory modalities with no apparent functional relationship to the imposed movement., (Copyright 1998 Elsevier Science B.V.)

Significant movement-induced gain changes in H reflexes have been observed in soleus muscle following passive movement of the lower limb. Hypotheses from these concepts were tested on magnitudes of H reflexes in tonically contracted tibialis anterior. From eleven subjects at rates of 20 and 60 r.p.m. passive leg movement, statistically significant attenuation from controls and phasic modulation occurred. The results make more general the conclusions from soleus H reflexes. However, the functional effect should be much smaller, as tibialis anterior H reflexes are smaller compared to those in soleus.

Passive locomotor-like movement induces depression of the gain of a fast conducting spinal sensorimotor path in humans. It was hypothesized that this gain control is mediated through a spinal circuit. In the first experiment, passive pedalling motion was rapidly initiated in eight able bodied subjects. Soleus H-reflexes (used to reveal the gain of the short latency stretch reflex) were recorded over the first 250 ms after the movement started. Significant depression in H-reflex magnitude was observed by 50 ms after the onset of movement. On the basis of the timing, this gain attenuation was likely mediated through a spinal circuit. In a second experiment we tested chronic quadriplegics with clinically complete lesions of the spinal cord. Of five subjects tested, three expressed the reflex and all three showed significant inhibition with passive pedalling movement (mean depression was to 39% of controls). Both the rapid onset of the gain change (Expt. 1) and the presence of movement-induced inhibition in individuals with spinal lesions (Expt. 2) provide evidence that this component of human locomotor control is located in the spinal cord. The initiating source is probably somatosensory receptor discharge due to the movement.

There is growing interest in the perturbation-evoked stepping response as an important element in the movement repertoire to maintain upright stance. It is possible that there is an important relationship between the well-documented, early 'automatic' postural responses and subsequent stepping responses. In this study, we characterize changes in early 'automatic' postural responses associated with the prior-planning and execution of a compensatory (i.e. stabilizing) stepping response. Seven subjects were tested on a 'moveable' platform which could translate in the anterior-posterior direction. The subjects' responses were analyzed for two different tasks: (1) 'constrained' (keep feet in place), and (2) perturbation-cued 'reaction time' stepping (step when the platform moves). Only responses to forward platform translations were analyzed in this study. Responses for each of these tasks were evoked using two different perturbation magnitudes. The higher magnitude was sufficiently large to evoke stepping responses in a large proportion of the 'constrained' trials despite instructions not to step. We compared the magnitude of the early evoked postural reactions between trials when subjects had pre-planned a stepping response against trials characterized by either (1) no stepping or (2) unplanned stepping responses. The results revealed that the early 'automatic' responses in tibialis anterior were always present in all subjects and all tasks. However, the evoked 'automatic' responses were approximately 36% smaller (over the first 50 ms) when followed by a pre-planned stepping response ('reaction-time' task). This task-related difference was similar for both perturbation magnitudes i.e. it occurred regardless of whether the 'constrained' tasks involved feet-in-place responses (small perturbations) or stepping responses (large perturbations).(ABSTRACT TRUNCATED AT 250 WORDS)

Our understanding of the postural control responses in the event of external perturbation has focused almost exclusively on the early automatic adjustments. The present study addresses another postural reaction that is functionally important: compensatory stepping. The purpose was to identify the relative importance by comparing the prevalence of compensatory stepping with and without instructions constraining the subjects' responses. Subjects stood on two force plates which were mounted on a "moveable" platform. Their posture was perturbed by the translation of the platform either forward or backward at various accelerations. Following a practice period, seven subjects each performed under two different tasks: "constrained" (keep feet in place) and "unconstrained" (no specific instructions given). The primary focus of the analysis was on responses to forward platform translations. Analysis revealed that the frequency of stepping tended to be higher in "unconstrained", as opposed to "constrained", tasks. The frequency of stepping was also related to the interaction between the tasks and the order in which they were given. Specifically, subjects stepped most frequently when they received the "unconstrained" task first. The frequency of stepping also increased as the magnitude of the platform acceleration increased. Time of onset of stepping, as defined from the force plate measures, began as early as 160 ms in one subject and averaged 250 ms across all subjects. These relatively fast response times suggest that step initiation often occurs well before the limits of stability are reached. A novel and unexpected finding was the identification of a third response type, intermediate to stepping and (bilaterally symmetrical) non-stepping responses.(ABSTRACT TRUNCATED AT 250 WORDS)

The research question was, do events arising from rhythmic passive movement of the human leg lead to inhibition of the H reflex pathway in the stationary leg contralateral to that movement? Further, as the angular velocity of the passive movement increases, does the contralateral reflex inhibition also increase? Stable stimulation of the tibial nerve elicited H reflexes in the EMG of soleus. Trials involved the stimulated or the contralateral leg being rotated passively in a pedalling motion, at various velocities. The controls were made with the subjects seated and relaxed. The results showed that reflex magnitudes were significantly depressed when the test limb was passively rotated at 60 rpm. in comparison to the seated control trials. Rotation of the opposite limb depressed reflex magnitudes in the test limb, which was stationary. This contralateral inhibition increased, (mean reflex magnitudes of 62.68%, 41.04%, 16.65% and 9.58% of peak-to-peak Mmax), as the velocity of rotation of the opposite limb increased (10, 30, 60, 90 rpm, respectively) (P < 0.01). The effect of movement velocity was interpreted as the result of altered sensory receptor discharge arising from the passive movement. It is concluded that contralateral sensory activity contributes to the movement-elicited afferent discharge which tunes the spinal somatosensory-motor mechanisms for human locomotion.

Modulation of soleus H-reflex magnitudes during pedalling, and their approximation when seated with appropriate joint positions and contractile activity was demonstrated in the previous paper. The present study investigated the modulation of H-reflexes during (A) pedalling movement in the absence of contractile activity, (B) different movement velocities and (C) movement of a single limb. Using a customized tandem cycle ergometer, seated subjects with trunk supported relaxed their leg muscles and allowed their legs to be rotated. Their feet were supported on the pedals with the ankle braced. Reflexes were collected at four phases in the movement cycle (with some at 13 phases) and with speeds of 5-60 revolutions per min (cycle times from 12 to 1 s). The results showed that (i) reflex magnitude substantially decreased with limb rotation (P less than 0.05). The degree of inhibition was dependent on the phase position. (ii) Increasing speed of passive rotation increased the inhibition at all positions, but was most pronounced near the fullest flexion of hip and knee. When subjects actively pedalled, the relationship between speed and inhibition remained. (iii) When the contralateral leg was moved and the target leg was stationary, crossed projection of reflex inhibition was clear. (iv) The reflex gain measured during active pedalling of one leg was similar to that observed during two legged pedalling. Again, a crossed effect from the contralateral leg could be observed. We conclude that the net influence of discharge from movement-elicited afference is inhibitory on this reflex path and that the reflex modulation during pedalling arises from overlaid sources.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of soleus H-reflex magnitude over a cycle of leg movement and the adjustment of controls to account for it were explored. During pedalling, H-reflex magnitudes in all nine subjects were highest in the power producing phase and lowest in recovery. Stimulation intensity was standardized. Compared to sitting, these reflexes were significantly depressed (P less than 0.05). The sitting condition was modified in one experiment, so that the angles of the limb joints and the contraction level of soleus were matched to their values, measured at 13 equi-spaced points, around the pedal cycle. This matching resulted in some modulation of the H-reflex around the pedal cycle, when sitting. When the contraction of tibialis anterior was added to these changes to the seated control, this modulation came closer to that seen during movement. Movement-specific modulation of the reflex was now harder to identify. These data raise the question of whether the three features presently used for matching are causative in the movement modulation of the soleus H-reflex and whether they represent effects arising from centrally descending or peripheral sources.

Inhibition of the magnitude of soleus muscle homonymous (H) reflexes occurs in humans when walking, compared to standing. The current study asked, (1) was the task modulation of Ia reflexes limited to soleus muscle, (2) was there support for attributing a presynaptic source to the inhibition in humans and (3) did an oligosynaptic short latency reflex show similar task modulation? In 3 subjects, H reflexes were evoked in vastus medialis and soleus, at 4 levels of contraction in the target muscle, with constant stimulus intensity when walking and standing. The reflex magnitudes in both muscles were significantly inhibited during the contractions for walking, compared to standing. Such inhibition also occurred in H reflexes of tibialis anterior muscle. An excitatory oligosynaptic reflex was then evoked in vastus medialis, through low intensity stimulation of the common peroneal nerve during walking and standing. The mean amplitudes of this reflex were not significantly different (P less than 0.05) between the two conditions, at any contraction level. The depression of quadriceps H reflexes, compared to the oligosynaptic reflexes through the same quadriceps motoneuronal pool in the same task, strongly suggested that the inhibition of H reflexes arose at other sites besides the motoneuronal cell body and proximal dendrites. We conclude that Ia H reflexes of various leg muscles of humans are inhibited when walking but that this does not generalize to the oligosynaptic short latency reflex between the anterior shank and thigh.

The paper describes electrical circuitry which replaces a mechanical brake, for perturbation of the contractile load of the legs during pedalling. The turning flywheel of an ergometer is connected to an alternator, with the electrical load provided by a power resistor connected across the output terminals. A 12-V battery provides the field current. Through variable resistors the field current is altered under microprocessor control, providing different steady-state loads when pedalling and also sudden transient changes of load. The point of change in load in the movement cycle can be accurately selected. The ergometer is instrumented for accurate measurement of pedal crank position, crank angular velocity and reaction force at the foot, to provide a physical description of the evoked steady states and transients, using microprocessor controlled sampling. The results show that the technique has application to the study of the range of reflex responses within a movement cycle and also to the more complex restoration of a movement pattern over a number of cycles. It is applicable to investigation of normal and diseased states.

Previous studies report that perturbing the posture of humans evokes specific patterns of muscular synergies in the legs. This study investigated the pattern of muscular responses of a whole limb when it was rapidly perturbed in the phase of extending during stationary pedalling. Subjects were instructed to resist. Accordingly, we anticipated increased extensor activity at knee and ankle to overcome the perturbation. This did not occur in the initial responses, appearing at latencies of 85-132 ms (mean = 104 ms). In contrast, there was facilitation in tibialis anterior, and the knee extensors vastus medialis and rectus femoris, together with profound inhibition of the ankle extensors soleus and lateral gastrocnemius. The anticipated extensor response across the limb appeared in the subsequent pattern of electromyogram (EMG) activity, with latencies ranging from 121 to 195 ms (mean = 168 ms), together with a large increase in propulsive force on the pedal. The difference in EMG patterns and latencies between initial and subsequent synergies was used to separate the responses into an earlier 'prevolitional' and a later 'volitional' component.